The present application is a national stage application of PCT Pat. App. No. PCT/US2012/031237, filed Mar. 29, 2012, which claims priority to U.S. Provisional Patent Application having Ser. No. 61/473,458, which was filed Apr. 8, 2011. These priority applications are incorporated by reference in their entirety into the present application, to the extent that it is these priority applications are not inconsistent with the present application.
Magnetic bearing systems are highly desirable in rotating machinery, especially turbomachinery, since they require no lubrication and have low friction losses. Magnetic bearings are, however, costly machine components and can be easily damaged during a shock event or any other type of transient overload event, characterized by large accelerations of the machine in radial and/or axial directions. Consequently, auxiliary bearings are required in most rotating machines and serve as a bumper to the rotor by preventing rotor impact with the magnetic bearings and thereby preventing severe damage. Auxiliary bearings, however, can only effectively absorb rotor thrusts and/or jolts to a limited degree of force before their use becomes inconsequential and the rotor nonetheless impacts the magnetic bearings.
To supplement the auxiliary bearings for severe rotor shock events, many rotating machines also employ “snubber” bearings. Snubber bearings serve essentially the same purpose as auxiliary bearings but are typically dissimilar in structure. Snubber bearings, for example, are typically offset radially and/or axially from the rotor and have at least one surface adapted to engage the rotor during radial or axial shock events (e.g., transient overload events). The purpose of the snubber is to protect the auxiliary and magnetic bearings during transient overload events which often create very large forces between casing and rotor and can result in the destruction of the auxiliary bearings and secondary damage to the magnetic bearings. Where the rotating machine is a compressor, surge is a typical transient overload event which can result in severe damage to any accompanying bearings if not mitigated by the use of a snubber bearing.
The snubber surface is often composed of a sacrificial metal or other material having a low coefficient of friction. Nonetheless, during a rotor transient overload event, the snubber surface can become severely galled and/or damaged. Moreover, the impact and duration of the transient overload event also causes the snubber bearing to heat up as contact between the snubber surface and the rotor generates a significant amount of friction. The heat generated from this contact causes thermal growth that reduces the operating life of the snubber bearing.
What is needed, therefore, is an improved snubber bearing and method of using the same in order to reduce friction and wear, limit heat generation, and prolong the operating life of the snubber bearing during rotor transient overload events.